Common medium multichannel exchange system



E. l. GREEN Dec. 22, 1936.

COMMON MEDIUM MULTICHNNEL EXCHANGE SYSTEM4 www L J jay. 5

Subscriber Subscriber i Zprafrvl H Selecting A Circuit Fb' .5 x v y ZI: Rezrgw INVENTOR E Z 9i/'een ATTORNEY Elter of Timed Cet Rect.

Dec. 22,1936. E l, GREEN 2,064,907

COMMON MEDIUM MULTICHANNEL EXCHANGE SYSTEM Filed May 26, 1934 4 sheets-sneet 2" n 1 -j-...J

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INVENTOR EJ.- @rem ATTORNEY Dec. 22, 1936. E, GREEN 2,064,907

`COMMON MEDlUMl MULTICHANNEL EXCHANGE SYSTEM INVENTOR E ZG/ceeI/v ATTORNEY Dec. 22, 1936. ,5. l. GREEN 2,064,907

COMMON MEDIUM MULTICHANNEL EXCHANGE SYSTEM Filed May 2e, 1934 4 Shania-sheet@ t Z y Egg/@ew Z0 BY a ATTORN EY V Patented 22, 1936;.

UNiTEnsTATEs PATENT? OFFICE- common MEDIUM MULrrcnANNEL AEmulation: srsrEM Estill yI. Green. East Orange, N. J., assignor to 'American Telephone and Telegraph Company. a corporation of New York In-the telephone exchange systems whichhave heretofore beenprovided it has been the practice to group subscribers in centralv omce areas. Each subscriber is connected to his own central ofilce by means oi an individual wire circuit or by a party line arranged for non-simultaneous use by a few subscribers. When a subscriber desires l to call another subscriber he utilizes his own line to the central oilce, and upon passing the desired number either to an operator lor to an automatic switching mechanism is connected to another line which extends either directly or via another olce or olces to the called party. For a system of this kind many thousands of subscriber circuits 'are necessary in each central oiiice are'aheThese are commonly provided inthe form of a network of multiple l,pair cables extending over the area. In accordance 'with the present invention it is proposed to substitute for such individual wire circuits extending to each subscriber a transmission. medium which is common to all subscribers and which will accommodatea range ofvfrequencies sumciently wide for a large number of telephone w: y els. Interconnection between subscribers is then carried out over these high-fre quency channels. t i i vThe essence of. the invention resides in making available to a large number of subscribers a transmission medium capable of accommodating a wide range of frequencies so that the subscribers may utilize, for the purposes of intercommunication, a plurality of signaling channels derived'from the wide-band medium. The provi- Y sion' ola transmission medium capable of handling the large number of channels required for an exchange system is, therefore. one of, the principal objects of the invention.

.This type of system is not known to have been heretofore invented, apparently because the necessary wide-frequency spectrum. which is required has not been available to the art either by radio or wire methods. vA conception of the,

-cently.

In the case of radio, until the recent opening at will 'and employed forl application may ze, 1934, serial No. '127,806 is einmal' (o1. 11s-1 5) band width which is required may be had by assuming that there is taken as a unit a local exchange system of 1000 subscribers and that each subscriber .is allotted an individual channel of 10,000 cycles, making a total band width of some 10,000,000 cycles. Any 'such band width. as this has ndt been available in the art until very reup of the ultra-high-trequency (ultra-shortwave). rangeythe art has not permitted the invention to be realized, for three reasons: First, l

there was not sufilcient space-in the frequency range, which until recently did not go beyond about 30 me., to accommodate this type of service along with all the other demands upon the radio spect second, the characteristics of the ,waves vary widely through this spectrum (up to 30 megacycles) so that -it is diillcult to obtain a lwide band having suiliciently uniform character-V istics for the purpose; third, the waves in this range do not out loil? verysharply withdistanoe, and as a result their interfering effects extend to great distances, in many cases thousands of miles, making it impossible to operate ya local service'- in one area without giving rise to interference in distance areas. .Withthe opening of the. l'iltra-high-frequency range, frequencies above 30 mc. (wavelengths below about 10 meters), an entirely newprospect is presented, making it for the rst time physically possible to set up by radio the system comprising .the invention. Of course, this is not saying that the system will either prove to be desirable or economical for.

actual operation. This will not be known until a great deal of development work is done and the generalart is much further advanced.

It will be appreciated also that the invention has not 'been feasible heretofore as a wire proposition because it has not been possible until very recently'to transmit over wire circuits for considerable distances frequency bands greater than some tens of thousands or, at most, hundreds of thousands of cycles. The transmission of bands of some millions of cycles as required by the invention has become possible by Wire methods only recently through the development 'of radically new types of wire systems, as exemplified by the coaxial conductor type of line, as described in U. S. Patent No'. 1,835,031, to L. Espenschied and H. A. Afel, and in a technical paper which is now being prepared and which it is expected will be published during 1934. These new wire systems are characterizedgby the provision of means for quite completely shielding the transmission path 1,5 .contained unit. In other words, in this wire em- Y J 2 from outside disturbances. This is done in the coaxial circuit by the outer conductor which serves both as a shield and one of the line conductors. By virtue of the skin effect at these very 5 high frequencies, the desired transmission is coniined to the inner surface of the tube and interference originating from without is confined to the outer surface.' Thus, the shielded type of circuit permits of blocking off a wide frequency band for the purposes of the inventionand subjecting (the transmitting medium' to full control in respect to the exclusion of interference, the prevention of overbearing on the part of outsiders, and the general knitting together of the systemas a selfbodiment of the invention there are retained the advantages of wire transmission, plus the advantage of radio in respect to the availability of a wide frequency spectrum.

In general, these recent developments, whereby the frequency range of radio and of wire transmission is being greatly extended, are the basis of the present invention. The extension of the frequency range-is so great as to amountto the imparting tothe art of a new dimension of 'development, thefrequency dimension, whereby it becomes possible to derive large numbers of telephone channelsand to handle them as a group. Thus, there is obtained on a frequency basis large numbers of identical channels comparable to the large number of physical circuits which are obtained in the local telephone plant in the familiar telephone cables.

While the use of radio as a common medium is v `contemplated within the scope of the invention..

this disclosure will be more particularly concerned with the provision of conducting or guiding media suitable for the transmission oi' a wide band of frequencies.

One form of such a transmission medium might be provided by a'network extending to all the subscribers within a given area. Cohceivably, such .a network mightvbe composed of ordinary wire circuits branched and interconnected so as to l make the common circuit available to each sub- The preferredv form of guiding transmission medium, however, is one which is digned for the transmissioni of a wide band of frequencies with comparatively low attenuation and pref'erably/ also. with shielding to minimize the eii'ect of external disturbances. Such a transmission mediurnl may be found in a circuit consisting of two conductors disposed coaxially with respect to one another; In a properly designed coaxial circuit,

as will be hereinafter explained, low attenuation at high frequencies may be attained by the emfrequency resistance andthe* use of a substantially gaseous-'dielectric between thefconductors.

With thistype of, circuit the outer conductor serves also as a shield whose protective eilectbecomes more` nearly perfectas the frequency is increased, so that at hightfrequencies practically complete immunity from external disturbances f` may be obtained.

y 'I he invention contemplatesalso as an alternadielectric.

ployment of -conductors of/suitably low highf tive to the coaxial circuit ahighfrequency transmission medium comprising a circuit consisting of two conductors surrounded by a shield. Here,

minimized by proper design `with substantially gaseous insulation and the thickness of shield may bedetermined so as'- to minimize interference` from external disturbances.

Yet another form of transmission medium suitable for carrying out the invention is a' dielectric wave guide comprising'a cylindriform dielectric a. conductor. .An advantageous form of such a transmission path isa hollow cylindrical conductor'containing air or some other gas'as the This type, of circuit is particularly adapted to transmit waves of very high frequencies, for example, of the order of a few centimeters or less in wavelength.

It is contemplated that the subscribers may be interconnected over the available channels either again, the high-frequency attenuation may be materialfwhich may orl may not be surrounded by (1) at a central cnice or (2) directly. Connection at the central oiilcemay be eected inmuch the same way as is now done by deriving out the channels into physical ,circuits which may be-- switched to one another.

pirect connection between subscribers is ef'- fected by a process of tuning or selection. This method takes advantage of the fact that the;

transmission medium is common to all of the subscribers, with all of the channels accessible to Y each subscribe at will. The. switching function is placed in e hands of the individual subscribers by eliminating the central ofllce in so far as connections within the given area. are concerned. Connection to subscribers in other areas is carried out through the respective central ofces which are joined by trunk channels or circuits suitable for the purpose.

In general, the invention contemplates the assignment to each-subscriber of a particular ire quencyor frequencies which might preferably be related to his telephone number.

ment of frequencies. should be such as to facilitate 'I'he assignthe interconnection oi' subscribers and various schemes of frequency allocation are provided in the invention forthis purpose. One feature ,of the invention is the supplying from a central point of the carrier frequencies required at the subscribers` stations.

. The foregoing outline having indicated some of the principles oi the invention as well as a few of .its major objects, the complete invention with its various details, features and purposes will now be understood from the following description when read in connection with the accompanying drawings, in which Figure l shows a schematic representation of the principal idea of the invention wherein a large number of stations are connected to a common transmission medium; Fig. 2 shows in schematic form a system for connecting subscribers through a .central office over a net# work of coaxial conductors; Fig. 3 shows one possible allocation of frequencies for a multi-channel exchange system: Fig. 4 shows a subscriber station arrangement which may be used in the system of Fig. 2; Fig. 5 shows a modulating device which maybe used in various applications of .the invention; Figs.'6 vand 7 in combination show a central olcearrangement which may b'e used in associationl withthe subscriber 'station ar- 'rangement of 4; Fig; 8 shows an arrangement of subscribers apparatusi for direct inter- '.connection without the assistance ofl a central station; Fig. 9 is a schematic diagram indicating es f ` how connections Imay be established directly between subscribers in the same area, while/connections between subscribers in diierent areas are completed through central offlces over trunk circuits; Fig. l is a schematic diagram of. a

guiding or conducting network which is available in common to a central station and a numberof subscribers; Figs. 1l, 12 and 13 show different forms of transmission paths which may b e used to form a-transmission medium common to a number-of subscribers; Fig. 14 shows an arrangement for providing central amplification in a network extending to al number of subscribers; Fig. shows a modification of Fig. 14 in which a number of subscriber stations may communicate with one another or with a central station over a -comrnon transmission network, the central station providing amplification as well as a common source of carrier frequencies; Fig. 16 shows one scheme for making a common transmission network available to a number of subscribers; Figs.

17, 18 and 19 show network arrangements alter.

' native to' that of Fig. 16; Fig. 20 shows a method of avoiding reflections at a junction of a number of branching circuits; and Fig. 211 shows an alternative method o f connecting subscriber stations 'to a common transmission network. 1

Referring to Fig. 1,` there is illustratedbne of the principal aspects of the invention. Inthis figure" is shown a common medium CM, to which areconnected a plurality of stations S1, Si, S3,-

etc., each of which includes transmitting, receiving and associated apparatus. This apparatus,`

7 which will be hereinafter described, is arranged to permit intercommunicaticn between different pairs of stations' over the common medium CM employing different frequency bands to permit simultaneous communication between diierent pairs of stations. Connectionbetween any pair of stations. may,f take place directly between the two stations as, for example; S1 and S3 or it may tak'e place through the common central station CS which is shown in dotted lines in the figure.

Fig. 2 shows-in schematic form a telephone exchange system in which a numberof subscribers may be interconnected through a' central oice,

the connection' between each subscriber and thel central ofce being accomplished overa network composed of coaxial conductors. Each subscriber is provided with transmitting apparatus-includingan oscillator-modulator and receiving apparatus including a demodulator, together with the neces? sary seletcing circuits. At the central olce av plurality of circuits such as thel one shown is provided fbr interconnecting between subscribers. Each connecting circuit includes two separate sets of apparatus, each of which comprises a transmitter and receiver, and a connection,v to which the operator has access, between the two sets of apparatus.

Transmission from subscriber A to subscriber B is eiected by means of the transmitter ST1,

the oscillator-modulator SM1, the tuned circuit selecting circuit SFa, the coaxial circuit'CA, the

selecting circuit OFs, the demo'dulator ODz, the hybrid coil'OHz, the connecting circuit CC. the hybrid coil OH1, the oscillator-modulator OMi, the selecting circuit OFz. the coaxial circuit CA,

the selecting circuit SF2. the demodulator SD1 and the receiver SR1.

It is proposed that different frequencies'be employed for the oppositely directed transmis`` sions in Fig. 2 and that the frequencies employed between the calling subscriber and the central oillce be different from those usedv between the central ofllce andthe called subscriber. Transmission might be on either a double' or a lsingle sideband -basis with the carrier .frequency transmitted.

In order that simultaneous conversations may take place between a number of pairs of sub scribers, it is necessary that the frequencies employed in the system of Fig. 2 be properly selected. One way of arranging the frequencies would be to assign to each subscriber a separate pair of frequencies, one for transmitting and one for receiving, and to provide apparatus at the Y central oce for receiving andtransmitting the various required frequencies. Thus, subscriber A might be assigned a transmitting frequency f1 and a receiving frequency f2, andsubscriber B transmitting and receiving frequencies la andf4, respectively. This `would mean that-in Fig.l2 the modulators SM1 and SM2 would operate with the modulators OM1'and OMQI-with carrier frequencies f2 and f4, respectively.

Withsuch ascheme it might prove desirable,

in order to facilitate the allocation of frequencies andthe interconnecting of subscribers, to'have a constanty `diierence between the transmitting and receiving frequencies of each subscriber or', in other words, to make f The frequencies f1, f3,A etc.; might then'. be'l grouped in one part of the frequency spectrum Y and the frequencies ,fn, f4, etc., in another part.

A diagram of a frequency allocation of this sort, assuming a total of 99 subscribers, is given in Fig. 3;- It will be noted that the order ofthe fre- `quency assignments corresponds to that of thesubscriber numbers. VThis scheme, while not essential to all embodiments of the invention, is

the central oihce in a scheme of this sort are illustrated'inFigs. 4, 6 and '1. While some of the apparatus shown in these and succeeding flgures is more particularly adapted to frequencies lying below the ultra-short Wave range, it will be understood that such apparatus is shown merely to illustrate -the principles of the invention, and is not intended in any `way to limit its scope. y d

The operation of the subscribers apparatus illustrated in Fig. 4 is as follows: When the subscriber desires to make a call, the receiver vSR1 is removed from the switchhook SH. Through cuit which furnishes power supply to operate the oscillator and modulator OC and-MO. The operationof the switch hook also provides D. C.

supply for the transmitter ST1, this supply being derived through a rectifier RF which is connected to the A. C. supply. The speech currents produced in the transmitter, after passing aoV carrier frequencies vf1 and f, Irespectively, and

.50. very desirable feature in many. 1, I

Detailed arrangements of apparatus which might be used at thev subscribers station and at the contacts of the switchhook this closes a ciry' through the vtransformer TR1, are modulated iis-Qn. the frequency. here assumed, to befi. .The oscillatorand, modulator may .be

y separatel umts of any desired types, or a self-l oscillating modulator might be used. The carrier and sidebands in the'modulator output, after passing through the selectingcircuit SE1, are conducted by the coaxial conductor CA1 to the centralr omce. When communication with the i central/cnice `is "established in .a manner herecentral oiiice, communication is established with" the called subscriber who is provided with apparatus identical with'that'of Fig. 4 except for the frequency assignments used.

inafte'rto be described, the number ofthe called subscriber 4is passed orally to the operator and` after'theproper arrangement 'aresfet up at the In the case oi an incoming call, the carrier frequency f2 and its associatedv sidebands are received over the coaxial circuit Cai, selected by the Jselecting"( :ir'euit SF2 and demodulated bythe demodulat'orjSDi; This demodulator may be of the vacuum tube type. in which case it should be kptalways in operating condition so that a call maybe received. Preferably, however, the \demodulator might comprise4V some passive element or elements, as, for example, copper-oxide units as shown ning. 5.. Associated with the demodulator is a relay RL1 which is.v operated by the rectiiied current resulting from the incoming carrier frequency and whichin turn operates the call bell SB. When the subscriber answers the call, the switchhook SH 'is operated, which,

` in addition to completing the contacts previously mentioned, `operates the slow-release relayRLa, which breaks the circuit of the bell SB. The

' received .voice-frequency currents, after passing through the transformer TR2, are applied to the receiver SRiand two-way communication is thus established. Il desired, a low-pass ilter LP (shown dotted in Fig..4) may be included in the receiver circuit to. suppress unwanted high-frequency components in the demodulator output. It will be understood that a similar illter may be provided in the arrangements 'hereinafter described. 'When the subscriber hangs up,l the slow-release feature of Ria prevents .his bell Vfrom ringing ,before the other party also hangs up.

' Figs. 6 and 7 illustrate central oice apparatus which maybe used in conjunction with the s ubscribers apparatus shown in Fig. 4. In the arrangement of Fig. 6 a number lof selecting cir cuits OFI, 0Faetc., corresponding to the transmitted frequencies of the diderent subscribers are connected toa common coaxial` conductor CA1.` The total number of such selecting circuits f is equal to the totg number oi subscribers to be served in the area.' Similarly,` selecting circuits OFz, OF5-etc., which correspond to' the receiving -trequencies ed tothe various subscribers areconnected tothe same coaxial conductor UA1. 4I1' desired, the transmitting and receiving apparatusmight Vbe grouped in other ways for connecv tion to the coaxial conductor network.

'I'he operation of the apparatus in Figs. 6 and "I is as follows; The incoming carrier frequency of a rcalling subscriber is received overthe coaxiakcircuit CA1, selected by one of the selecting circuits, for example OFi, and demodulated by the demodv 'm .ulator ODi. Associated withthe demodulator is a relay OR; whose winding isconnected to the demodulatoroutput through the contacts of an-` otherrelay ORn. The Vrectified current resulting vfromthe incomingcarrierirequency operatcsthe line relay ORi, which in turn lights the line lamp ers station being shown .the subscriber OL1, thereby calling` the attention of thev operator to the incoming call.

The operatorgis provided with a number of cord circuits of the type shown in Fig. 1. each cord circuit terminating in plugs, such as OPi and OPs'. Upon observing the vI hie lamp'she inserts the,

plug OPi associated with 'oneof the cord circuits into the jack OJ; of the calling subscriber.:

Thus the voice-frequency currents delivered by the demodulator, Aafter passing through the hybrid coilOHi, reach the operators set OS.

The sleeve connection of thel plug operates the Y relay OR: which extlnguishes the line lamp OLi, and connects power supply tothe oscillator-modulator OM1.- .The insertion of the plug operates the supervisory relay ORs, which lights the supervisory lamp Olin.A This lamp remains lit as long as the calling subscriber has his receiver o the hook. The condensers OC in the cord circuit serve to interrupt the D. C. path,'so` that line and supervisory relays and lamps may be similarly operated at the otherside oi' the connection.

To the conjugate terminals of the hybrid coil OH1 there are connected the oscillator-modulator OM; and the selecting circuit OF; s that the operator may answer the incoming call. The.v

subscriber then passes the number of the called party to the operator. A busw test of the called subscribers line is obtained through the tip ot the plug at'the other end ofthe cord circuit and the sleeve of the subscribers jack. In case another cord circuit is already connected to a jack corresponding to the number of the called party, the sleeve of the jack wiilhave battery connected ,to it'and a click will be produced in the o rators receiver. A

If t e called subscribers line is not bl1SY. the

operator places the plug CP2 in a jack such as 0J: of Fig. 6 of the called party, thereby completing the connection. 'I'he operation of the relay ORS connects power supply to the oscillater-modulator CM2, so that the carrier is transmitted and thebell of the called party rings. When the calledsparty answers, the supervisory lamp OLs` is lighted by means of remains lighted until the called party hangs up. 'I'he conversation then proceeds withJthe apparatus functioning in a manner similar to that described in connection with Fig. 2. When the conversation is completed, the lamps OL: and OLe are extinguished as the subscribers hang up,

relay ORA and and the operator Vtakes down the connection.

Bymeans oi keys, such 'as 0K1, the operator may connect her set to diiierentcord circuits, and

using the key 0K1 she may split the connection and talk toieither subscriberwithout the other.

The arrangements Ythus far described hge been designed for the interconnection of s may be connected with one another by a direct scrlbers through acentral ofl'ic'e. There willnow' be described arrangements whereby subscribers path, without the assistance of a'central con;

.necting station.'

One possible arrangement of this kind is /illus-M- trated in Fig. 8, the apparatus for one subscrib- 65 diasrammaticauymthis of the arrangementshowny in' l 2,064,907 to the number of the called. party. This switch may be of various typesjand may comprise several switches rather than a single one. The ad justment of HS mechanically adjusts the midband frequencies of the variable selecting circuits VF; and VFa to the receiving and transmitting frequencies, respectively, of the called'party, and

also adjusts the variable condenser VCr to a value such that the frequency of the oscillator relay or may be delayed in operating due to the i riable condenser VC1.

, cenversation.

C corresponds to the receiving frequency of thel y called party.

Having set up the number of the called party,

the subscriber removes the receiver SR1 from the switchhook SH. The operation of the switchhook provides power supply for the oscillator and modulator OC andMO. In addition, the operation of the switchhook energizes the slow-release relay RLa whose winding is connected to the D. C. supply through a back contact of the relay RLr andra make contact of the switchhook. the time the switchhook is removed therelay RL1 in the demodulator circuit is deenergized since vthe demodulator is connected to the fixed selecting circuit FFz, which is used for incoming calls.) The relay RLa in pulling up establishes for itself a new circuit leading direct through one of its make contacts to the switchhook` contact and thence to the D. C. supply. The relay RLa then remains operated until the calling subscriber hangs up at theend of the conversation.

'I'he operation of RLa connects the modulator MO to the variable selecting circuit VF1, the de- 'modulator SD to the variable selecting circuit]- VFz, and the oscillator tuned circuit to the vay If at this time the transv mitting frequency of the called partyfis busy, this frequency will be passed by theV selecting circuit VFz and rectified in the demodulator SDi,

. so that the relay RLi will be operated. This in turn will energize the relay RL4, which is con nected through a make contact of RL, a back contact of RL2' (which may be a slow-operating time required for the oscillator to become operative) and the make contact of RL; to the D. C. supply. The operation of RL. will serve tovlight the busy lamp BL, thereby indicating to the subscriber that the number of the called party is busy.

If Vthe called partys number is not busy, the relays RL; and RL4 will not become operated and the carrier supply from the oscillator OC will be connected to the modulator'MO and the rectier RFL through a back contact of Bla. The

. current from the rectifier RFz will operate the relay RLe, thereby opening the circuit of RL4. The transmitter ST1 will bel connected tothe D. C. supply through a back contact of RL; and they circuit of the receiver SR1 will be completed through another back contact of RL4.

The receiving frequency ofthe called party, which is sent out by the oscillator 0C, will ring the bell at the calledhstation (in a manner to be described later) When the calledparty answers, his transmitting carrier frequency will be received by the demodulator SD1 and the relay RLr will be operated, but this will not aect theother circuit connections* The circuit, is then-in readiness for the two subscribers to .proceed with their When the calling subscriber hangs up, his' equipment returns to the condition forreceiving an incoming call.

The'voperationf of the-arrangementof Fig. 8

in receiving an incoming call will now be de-- lvscrlbed. The demodulatcr SD1,

if composed of a passive element or'elements, will be continuously in operative condition. 1f a demodulator of the vacuum tube type is employed the tubes should be supplied with power continuously or at least during the period when itis desired to receivej calls.

When the subscribers receiver is on the switchhook the relay RLa is deenergizcd. The demodulator SDI is connected through a back contact/of 'this relay to the fixed selecting circuit FFz so that the incoming call may be received. The

modulator -MO is connected through a back contact of RL: to the ilxed selecting circuit FF1, while the oscillator is properly adjusted to the subscribers outgoing frequency assignment by the fixed condenser FCI.

The rectified current produced by the incoming carrier frequency operates the relay RLi, closing the'circuit of the bell SB which is-con nected through one of the contacts of the switchhook. When the subscriber answers the call, the bell circuit is broken by the switchhook. At

the same time the power supply is connected to the oscillator and modulator through a contact of the switchhook and carrier supply is brought from the oscillator-to the modulator through a back contact of RIA (which is deenergized). The

rtransmitter receives battery supply through a back contact of RL4, while the receiver circuit is completed through another back contact of RIA. The apparatus is therefore in condition for conversation th the calling party. At the conclusion of th conversation, the subscriber hangs upl and the apparatus is in readiness for the next ca l.

As has been explained, the subscriber station arrangement of Fig. 8 is designed to provide a direct connection between the subscribers within a given` area. It is proposed to interconnect such stations with subscriber stations in other areas by providing in each area a central olce, so that a connection may be estab- .lished between a subscriber and his central oillce, using a frequency or frequencies assigned for that purpose, andthe connection completed over suitable interofce trunks. The method is illustrated schematically in Fig. 9. The central oiilce apparatus to be used in carrying out this method might be patterned after that shown in Figs. 6 and 7. The types of trunks which may .be used to interconnect the central oiiices are scribed for selecting channels at subscriber stations and central omces, the Ventire selection has been accomplished at the channel frequency. It will be evident that this method of selection might be replaced by the well-known superlieterodyne method, with 'partial selectivity provided at the channel frequency and the remaining selectivity furnished by a sharp, interme- `-diate frequency selectingV circuit or filter.

It Vwill be evident to those skilled in the art that arrangements for automatically controlling I the volume 'of the signals delivered by the scriber's equipment may be incorporated in any of the subscribersstation arrangements which have been described, so that conversation bes tween diil'erent subscribers may take place over mon transmitting medium. Circuits o! this type` will now be more fullyy described as well es other arrangements for providing 'a medium capable of transmitting a wide band of frequencies.

Probably the simplest form of common medium would be a network extending to all sub; 'scribers within e." given area and to the central oice for that area. Such a network would be one comprised' of ordinary two-conductor circuits suitably interconnected and branched so lthat the common circuit will be available at ail desired points. The circuits comprising the network might, for example, consist of pairs oi open-wire or cable conductors. A network of this kind is shown schematically in Fig. 10,

where each line 'rents a pair of nductors,A

C designates the 'central vomce and S a suhscriber's station.

It is contemplated also in accordance with the invention t the wire network employed for distributing light and power currents might be used as the common medium from which' to derive high-frequency channels for the phone exchange system. The high-frequencychannels would be superposer! upon the power network by carriermethcds which are well known inthe art.

Another Afprm of on. medium,'and

v one which isoccunaslyv dvstaseous in? that it ls capable oi' itting 'a wide band.v of irequencies with comparatively low attenuation and which may be soshieldedas to be practically immune to external disturbances. may be found in 'a circuit consisting of two conductors disloed coaxially `with to one another. Such a circuit has been disclosed inthe patents to L. Espenschied andRA. Ael, No. 1,835,031. De-

e cember 8, 1931, H. A. Aiiel and E. I. Green, No.

1,701,092, November 1d, 1930, etc. In this form u maybe obtained bythe enployment o f conductors whose high-frequencyreslstance is suitably srnall and by the employment oi a sub` stantially gaseous dielectric outer conductor .provides shielding against external disturbances.' which shielding becomes more nearly perfect as theirequency is increased, so that at high frequencies the noise due to thermal agita` tion in the conductors becomes the factor which determines the minimum transmission'level. A diagram .of one form ci coamal circuit isgive'n in Fig. 11, wherei `designates an outer cy lindrical conductor'and! a conducting wire or 70 .tube concentric therewith. 'lhe' two conductors 4are maintained in proper spaced relationship by the, insulating disks l. The. circuit is shown connectedtoapparatusl." As an alternatlveralto the' coaxial circuit.' the This, oi' course. is a verydesirable i'ea-V of circuit, low attenuation at' highirequencies accesos sub- .L

prise a network of circuits, each consisting ci two parallel conductors surrounded by a shielda Circuits or .this type aredisclosed intheapplications ci 'j Green,Curtis and Mead, Serial No. 674,762, Patent No. 2,034,032, Green and Curtis, Serial No.

' 674,763', Patent No. 2,034,033 and Green 'and Leibe,- Serial No. 674.764,'Patent No. 2,034,034, all led June?, 1933 andall vissued March l'I, 1936. One form of such' a shielded pair is illustrated in Fig. 12, where I designates a conducting shield and 2 and 4 represent conductors located on opposite sides of'and equidl'stant from the axis oi the shield. These conductors are heldin proper spacedlelatiou to one another and the shield by means oi' insulating disks 3. The conductors 2 and l may be connected to apparatus i5. In a circuit of this type it is desirable to employ conductors of suitable size, with as far as possible gaseous insulation, in order to minimize .the high-fie quency resistance and capacitance andthereby reduce the high-frequency attenuation.' The' thickness of meld, moreover-,should be such as to minimize exte disturbances.

The invention contemplas also utilizing as the high-frequency transmission medium a dielectric wave guide `comprising a cylindrii'orm dielectric material which may or may not be surrounded by a conductor. Such dielectric guides are disclosed in the patent applications of G. C. Southworth, Serial No. 661,154,.led March 16, 1933,y and Serial No. 701,711, illed December 9, 1933. An advantageous form of such a wave guide is a hollow cylindrical conductor containing air or l some'other gas as the dielectric. This type of 'circuit is particularly adaptedY to transmit waves ci very high frequencies, for p t ofthe Fig. 13 shows/one i of such Aa. ve guide. Apparatus d prcduces highfrequency signaling currents which` are propagated along the coaxial circuit g the tube l and the conductor 2, separated from one another lby insulating 3. as shown in the cut-away,porticn ci the tube.

If the frequency is sumciently high. the conduc.

Y- tor ,2 need continue only a short distance inside thetube l, and the electromagnetic waves from order of afew centimeters or less in wavelength.

the apparatus d travel along conductors i and 2 and will continue along the tube i where the inner conductor has been removed, traon taking place on the inner surface of the tube and the'enclosed dielectric.J A similar device may be used at the other end oi' the dielectric wave guide to receive the signals.

It is contemplated in accordance with the invention that any ofthe subscriber-s set and central oillce arrangements which have been described ,may be employed with any of the above types of traion paths and, 'in general. theconnections between subscribers may-be en'ected either directly or via thevcentral cnice.4 When using a network such as that of Fig. 10 for a system in so allocating the frequenciesthat the outermost subscribers utilise the lower frequencies for 'which .which connections are established' through the central omce, there might be some advantage in theattenuations are smaller, while the close-in subscribers utilize higher frequencies which under go higher attenuatioris.`

A conducting network, lin addition to being I .capable of use with various arrangements which are adapted also to radio use, makes it-possible to employ certain schemes in the case of radio. deilnite'ion network permits the use of which are not feasible Y balancingmethods to separate opposite directions Thus the emstence of a' tsrA of -transmission and permits, for example, in a systemin which transmission takes place directly betweensubscribers, amplification of the transnais transmitted by any one subscriber are amplipedance'. It is well known that the characteristic impedance of a circuit at high frequencies is substantially independent of Y frequency and approaches a pure resistance of value JIT? where L is the linear inductance and C the linear capacitance of the circuit. Reflection effects which would resultl in standing waves on the circuit are avoided by the terminations and the high impedance connections to the subscribers sets.

At an intermediate point in the circuit there is inserted the hybrid coil HB, to the conjugate terminals of which are connected the input and output of the ampliler- CR which is designed to amplify a broad band of frequencies. Thus the sigfied at the ,central point and passed on to the other subscriber.

Fig. 15 shows a modification of Fig. 14 wherein coaxial lines extend in several directions from the central point. Each line is terminated in a resistance RT which approximates its -characteristic impedance. At `the central point each of'the inner conductors of the various branches is brought through a resistance JR to a common Ajunction point. The function of the resistance JR is to avoid high-frequency reflections which otherwise would be produced at the junction. The

method of proportioning these .resistances will be Ihereinafter explained. The hybrid coil HB is shown connected to the common junction through a resistance JR, though it would be possible to omit this resistance if a suitable value were assigned to the others. The opposite side of the hybrid is terminated in a resist-ance BR which is equal to JR plus the parallel impedance of all the branches. The various transmissions arriv-y ing at the central point are amplified by the amplier AM and retransmitted.

In addition, the carrier frequencies required by the subscriber are supplied from the source CS at thecentral point. Thus Fig. 15 might'be used 'in conjunction with the subscriber station arrangement of Fig. 4. The arrangement of Fig. 15, including the provision of a common carrier frequency supply, `may be used also with the other subscriber station arrangements. The central ofce apparatus for communication with subscribers is shown at CA in Fig. 15. In case amplifica 'Zion at the central station is not required, the

carrier source CSrand the apparatus CA may be connected directly tothe line junction.

There will now be considered some of the schemes which may be followed in disposing the network which forms the common transmission medium so that it will be available to the subscribers in a given area as well as to thecentra-l station. If the subscribers are located in an urban district, the development of the buildings and streets will very likely be in the form of more or less rectangular blocks. Preferably the network should conform to this plan.

Perhaps the simplest scheme for this purpose is-to arrange the network in the form of a grid as illustrated schematically in Fig. 16. In this gure each line represents a conducting circuit or guide (for example; a coaxial circuit), the various circuits being connected together at points indicated by the dots. The equipment of each subscriber may be connected to the nearest point of the grid through a high impedance, as shown, for example, in Fig. 14. The central station apparatus CS is connected to the grid somewhere near its center.

One possible difculty rin the arrangement \of Fig. 16 may arise from the large number of points 0f junction or branching which it contains. Each of these points represents a source of transmission loss due to the division of energy between the branches, and also. unless the junction-points are specially treated, a source of high-frequency relections which may be produced by the improper termination which the junction may oier to each circuit entering it. f

A further diculty' may arise due to the fact' that between any two points of the grid there are several 'paths' over which transmisison may take place, so that the relative phases Aofthe signals arriving over the different paths may result in very inemcient transmission for particular frequencies between particular points.

Another type of network arrangement is shown in Fig. 17. Here a single transmission circiut is provided which extends outwardly from the cen-Y tral station in what may be termed a square spiral, the circuit being terminated at its outer end in a resistance RT which is substantially equal to its high-frequency characteristic impedance. Again, each subscriber may be connected to the nearest point of the network, the length of such connection, however, being on the average somewhat greater than for Fig. 16. It will be evident that the scheme of Fig. 17 eliminates the points of branching which appeared in Fig. 16 and that the total network mileage has been reduced.

Yet another possible arrangement is shown in* Fig. 18, this arrangement being similar to the grid of Fig. 16'but having fewer points of branching. The ends of the various circuits may be terminated in resistances RT as in Fig. 16. It would evidently be possible by bringing the various paths back to the central point to obtain a scheme having only a single junction point rather than the multiple junctions as shown.

.Another type of network arrangement is shown;

in'Fig. 19, the circuits being arranged in the forni of main Ffeeders extending outwardly from thecentral station and branch feeders extending at various points from the main feeders.

The/reflection effects which would appear at a junction of several circuits such as those in Figs. 16, 18 or 19 may be avoided by the method illustrated in Fig. 20. Assume a number of circuits n which are toghe-connected to a common junction. For convenience in derivation a single wire ground returnr type of circuit may be assumed. Let each circuit have a characteristic impedance Zp. each circuit are avoided by termination approximately equal lto Znas shown. Each circuit is then connected into the junction point through an impedance Z1 whose value isdetermined as Reflections at the outer end of follows. The impedance f looking junction'from any one circuit is 'The impedance Z1 will be substantially a resist? ductOr.

intowardthe +20 Z1+T 1 (2) Reflection is avoided by making (2) equal to Zo. which upon simplication yields the following relation: 2'

ance at high frequency, For a coaxial circuit Z1 would be connected in series with the inner con-V Fora balanced two-wire circuit Z1/2 A would be inserted in series with'e'ach conductor.

' It will be `apparent that by'terminating the ends of circuits, in the high-frequency characteristic impedance and by treating the junctionsin accordance with Fig. 20, the networks of Figs. 16, 1 8 or 19 may be arranged so that vthe high-fre.-

quency impedance looking in any direction 'from any pointin the network will be substantially constant and equal to the characteristic impedance of the type oi' circuit which constitutes the network. In general; this result will obtain at frequencies above the voicerange. Thus transmission between any two'points in the network may be accomplished without reflection eiects. 1

The. method of Fig. also makes it possible t0 connect subscriber stations to a common transmission network without incurring the loss involved in a highimpedance connection such ash Y shown in Fig. 14. Y In the arrangement shown in Fig. 21- the equipment SA ofeach subscriberis designed tchave a high-frequency impedance equal to the high-frequency characteristic impedance of the line circuit, and reilections at circuit junctions are avoided by inserting resistances n-.2 Y l' f Zi--n While the entire p discussion been restricted to systems wherein a common transmission medium is employed for the provision of telephone exchange service between subscribers, itwill be understood that the invention contemplates the use of the same general methods for other types oi.' communication. In particular,

, it will be seen that there has been disclosed a medium which is capable of handling a wide band ot'frequencies 'from which a substantial number of televisionchannels may be derived.

It willturthermore be obvious that the general t principles herein disclosed may be embodied in ferent pairs o! stations..

.many other organizations widely -dlfierent from those illustrated without departing from-the spirit of the invention as deiined inthe following claims. What'is claimed is:

1. In a system'fo'r the exchange of intelligence 'between any of a pluralityv of stations taken in pairs, a single common transmitting medium y interconnecting all ci' said stationafsaidmedium being arranged in the form of an extended grid' fpairs. al on network vto which of acceso? said stations is'connected', having a plurality of points at which circuits are branched and a plurality of points at which circuits are terminated, means at saidpoints to equalize the high-frequency impedance measured atany point in any circuit 'of the network looking in either-direction, and means whereby said networkmay be utilized simultaneously for va. plurality of communications between .diffrent pairs of stations. i 3. In a system for the simultaneous exchange of intelligence between any of a plurality of stations taken in pairs, a transmlssionnetwork to which eachof saidstations is connected composed of circuits all having Isubstantially the same high-frequency characteristic 'impedance and having -a plurality of points at which circuits are branched and a. plurality of .points at which circuits are terminated, means at said points to equalize the high-frequency impedance measured at any point in any circuit of the network looking in either direction. Y

4. In a-system for the simultaneous' exchange of intelligence between any of aplurality o fjstations taken in pairs, a transmission network' towhich each of said stations is connected com- 'posed of circuits all having substantially the same high-frequency characteristic impedance and having a'plurality of points at which circuits are branched and a. plurality of points at which circuits lare terminated, means at3 said pointsl to at any point in any' circuit of the network looking in either direction, and means whereby-said net .work may be utilized' for a plurality of exchanges of intelligence between diierent pairs o'f stations.

equalize the high-frequency impedance measured 5. In a system for telephone communication "between any of aplurality' of stations taken in pairs, a transmission network to which'each' oi saldstations is connected composedof circuits all having substantially \the same high-frequency of points at which circuits are branched and a 40 characteristic impedance and having a plurality plurality of pointsat which circuits arertermiv nated, means at said pointsv to equalize the'high- -frequency impedancemeasured at any point in 'any circuit of the network looking in either direction, and means whereby said network may be utilized simultaneously for a plurality of tele'- phone communications between different pairs of stations. Y y y.

6. In atelephonesystem for simultaneous com` munication between any of a plurality of stations taken in pairs, a transmission networkcomposed n 'of circuits all having substantially the same high. frequency' characteristic impedance ofvvalue Zo, I to which each of said stations'is connected. Sfaid network including a plurality of circuit an impedance 4substantially equal to n--Z A 20(7) connected in series between each junction of vn circuits andY each circuit entering said junction, and an impedance substantially equal to Zo` connected to vthe endof each circuit which joined to another'circuit. Y

7. In 'a system for telephone. communication between any of a plurality of stations. taken-in pairs, 'a transmission network composed of circuits all having substantially the same highfrequency characteristic impedance ot value Zo,

each of said Vstations jbeing connected to said junctions.

is not f network. said network including a'plurality of 75 vio circuit junctions, an impedance substantially equal -to n--2 l l 2"( 'n connected in series between each junction of n circuits and each circuit entering said junction, and an impedance substantially equal to Zo connected to the end of each circuit which is connected in series between each junction of n circuits and each circuit entering saidjunction,- and an impedance substantially equal to Zu connected to the end of each circuit which is not joined to another circuit, and means where-` by said network may be utilized simultaneously for a plurality of exchanges of intelligence between diierent pairs of stations.

-9. In. a system for the exchange of intelligence between any of a plurality of stations taken in pairs, a. transmission network composed of circuits all having substantiallythe same highfrequency characteristic impedance of value Zu, each of said stations being connected to said network through an impedance which is large in comparison with Zo, said network including a plurality of circuit junctions, an impedance substantially equal to zii?) f connected in series between each junction of 1|. circuits and each circuit lentering said junction, .and an impedance substantiallyequal to Zo connected to the end of each circuit which is not joined to another circuit, and means whereby said network may beeutilized simultaneously for a plurality of exchanges of intelligence bei tween diierent pairs of stations.

vinterconnecting all 10. In a system for the simultaneous exchange of intelligence between any of a plurality of stations ftaken in pairs, a transmission network of said stations having a plurality of points at which circuits are branched, and means at said points to prevent reflection eiects at any frequency above the audible range in transmission between any two stations.

11. In a system for the simultaneous exchange of intelligence between any of a plurality of stations taken in pairs, a transmission network including a plurality of points at which circuits are branched. apparatus at each of said stations connected to said network, and impedance means placed atproper pointsof said network to'prevent reflection effects at any frequency above the audible range for transmission between any two stations. i

12. In a system for the simultaneous exchange of intelligence between any of a plurality of stataken in pairs, a. transmission network ex-A tending to all of said stations of circuits all having substantially the same high-frequency characteristic impedance of value Zu, and including a plurality of circuit junctions, an impedance substantially equal to connected in series between each junction of An circuits and each circuit entering such junction,A and an impedance substantially equal to `Za connected to the end of each circuit which is not joined to another circuit, said network having the property that transmission between 'any two stations may take place substantially without reilection effects at any frequency above the audible frequency range.

13. As a common medium for signalling from any one of a plurality of more than two stations to any one other station, a system of conductors to which said stations are connected and-having branching connections one conductor to another,l

in combination with means to equalize the impedanceeither way along any path from one station to another station.

14. The method of joining more than two cir- 7' cuits each of which has a characteristic impedance of value Zo which consists in inserting between each circuit and the common junction an 'impedance equal to Zu. (1L-2)/11. where 11. is the number of circuits joined together.

, 15. A transmission network composed of circuits all having substantially the same high frequency characteristic impedance of value Zo, said network including a plurality of ,circuit junctions and a plurality of points at which circuits are terminated in substantially characteristic impedance and an impedance substantially equal to Zo (1L-2) /n connected in series between each junction 'oli n circuits said junction. y l

16. A transmission network composed of circuits all having substantially the same high frequency characteristic impedance of value Zn, said `network including a plurality of circuit junctions Vat which circuits are branched and a plurality of points atwhich circuits are terminated in substantially characteristic impedance and an impedance substantially equal to Zo (1L-2)!nconnected in series between each junction of n circuitsand each circuit entering said junction,

and apparatus connected to said network at each of /a plurality of points through an `impedance which is large in comparison with Zo.

17. A transmission network composed of circuits all having substantially the, same 'characteristic impedanceand having wa plurality of points at which circuits are branched and a plurality of points at which circuits are terminated in substantially characteristic impedance.

--and impedance equalizing means at 'each branching point so designed' with respect to the circuits branched therefrom that the impedance measured at any point in any circuit of the network looking in either direction is substantially equal to the characteristic impedance. of each circuit.

18. A transmission network composed of circuits all having substantially the same high frequency characteristic impedance of substantiallyv constant value and having. a plurality of points at which circuits are branched anda plurality of points at which circuits are terminated in high frequency characteristic impedance, andimpedance equalizing means at each branching point so designed withrespect to the circuits branched and each circuit lentering therefrom. that the high frequency`y impedance 35.

measured at any point in any circuit of the netan impedance substantially equal t Zo .(n-2) /n Lwork lookingin either direction is substantially connected in series between each junction of n independent of frequency. y circuits and each circuit entering said junction, 19. A transmission network composed of cirand an impedance substantially equal-to Zn concuits all'having substantially the same high frenected to the end of each circuit which is not 5 quncy characteristic impedance of value Zo, said joined to another circuit.

ne work including a plurality of circuit junctions, ESTILL yI. GREEN. 

